Re: [PATCH v14 1/6] sched/core: uclamp: Extend CPU's cgroup controller

[Michal Koutný]

On Mon, Sep 02, 2019 at 04:02:57PM -0700, Suren Baghdasaryan <surenb@xxxxxxxxxx> wrote:
> > +static inline void cpu_uclamp_print(struct seq_file *sf,
> > +                                   enum uclamp_id clamp_id)
> > [...]
> > +       rcu_read_lock();
> > +       tg = css_tg(seq_css(sf));
> > +       util_clamp = tg->uclamp_req[clamp_id].value;
> > +       rcu_read_unlock();
> > +
> > +       if (util_clamp == SCHED_CAPACITY_SCALE) {
> > +               seq_puts(sf, "max\n");
> > +               return;
> > +       }
> > +
> > +       percent = tg->uclamp_pct[clamp_id];
> 
> You are taking RCU lock when accessing tg->uclamp_req but not when
> accessing tg->uclamp_pct.
Good point.

> Is that intentional? Can tg be destroyed under you?
Actually, the rcu_read{,un}lock should be unnecessary in the context of
the kernfs file op handler -- the tg/css won't go away as long as its
kernfs file is being worked with.

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